The field of the invention relates to air/fuel control systems.
It is known to adjust the engine air/fuel ratio during open loop air/fuel control conditions. Under such conditions, fuel is delivered to the engine independently of feedback control. Otherwise, fuel delivered to the engine is adjusted by a feedback variable generated from an exhaust gas oxygen sensor.
It is also known to run the engine lean of stoichiometric combustion under open loop conditions for various reasons. One example is to provide more rapid warm-up of the catalytic converter during cold engine operation by creating excess oxygen in the engine exhaust. Another example of lean operation during engine warm-up is disclosed in U.S. Pat. No. 5,483,941. This particular patent describes cutting off fuel to a preselected number of engine cylinders to reduce engine torque during traction control. During cold engine operation, the air/fuel ratio of the remaining cylinders is run lean of stoichiometry. Otherwise, the oxygen pumped through the deactivated cylinders may mix with unburnt hydrocarbons resulting in an exothermic reaction at the catalytic converter.
The inventors herein have recognized numerous problems with all the approaches described above wherein lean air/fuel operation is attempted under open loop air/fuel conditions. For example, the inventors herein have recognized that the open loop lean air/fuel calculation may be too lean resulting in engine stumbling.